The present invention relates to authentication, and, in particular, to the authentication of visual symbols.
Authentication is the verification of the identity of a person, object, or process. In a communication system, authentication verifies that messages come from their stated source. In an access control system, physical access may require authentication by photo ID, password, or biometric data. Paper document authentication is often made by checking a handwritten signature. Credit card transactions involve PIN or signature to authenticate the cardholder.
With growing threats from both crime and terrorism, authentication becomes more and more important, and the authentication methods need to be more and more robust to withstand sophisticated attacks. Thus, for example, a password transmitted over an unsecured network is not considered reliable for authentication, because it can be intercepted and replayed by an attacker. The science of cryptography offers well-known methods such as challenge-and-response, encryption, and digital signatures, to support authentication that is sufficiently robust to withstand practical attacks.
Another relevant aspect with respect to the present invention is machine-reading of visual symbols representing information or data. A common non-limiting example of visual symbols is a block of printed text, which requires a sophisticated OCR (optical character recognition) algorithm for reading by machine. To facilitate machine-reading, special fonts have been developed, as can be seen on the bottom of bank checks and in the type used to emboss credit cards. To further facilitate machine-reading, the one-dimensional and two-dimensional bar-codes, and a circular color code have been developed. The two-dimensional bar-code offers higher information density and error-correction capability, while the circular color code, such as the one described in PCT publication WO00/04711, has proven to be easily-identifiable within large images, where a multiplicity of tags bearing such a code is embedded within an image that contains additional information, such as a digital photograph of a crowd.
Visual symbols appear on product packaging or are printed on tags attachable to a product, document or person. The machines used to read visual codes are special-purpose laser or CCD scanners, or general-purpose digital still or video cameras that acquire an image containing the visual code and send it to a computer for image processing.
A major advantage of prior-art printed visual codes is the ease and low cost of production. However, this very advantage makes such codes also vulnerable to unauthorized duplication. Various efforts have been made to make such duplication harder, for instance by covering the printed code with an opaque layer that is seen through by infrared scanners. However, none of the methods of the prior art offers robust protection against a sophisticated attacker who uses affordable, off-the-shelf equipment to read and reproduce the symbols.